Mechanisms of antidepressant drug actions are not clearly understood. In the present application, we propose that antidepressant drugs work via a classical trophic mechanism, i.e. proliferation of terminal branches (sprouting) of central monoamine axons in cortex and the limbic system. We also propose that the molecular signals mediating these structural effects are neurotrophins, e.g. for the serotonin system, the critical signal is brain-derived neurotrophic factor (BDNF). Our experimental design takes a stepwise approach using cellular and molecular methodologies, including transgenic mice. We first evaluate the ability of serotonin (5-HT)- and norepinephrine (NE)-promoting compounds to cause 5-HT and NE sprouting in selected areas of neocortex, hippocampus and the subcortical limbic system. In parallel, we evaluate the role of antidepressants in enhancing neurotrophin expression and transduction in the same areas, focusing on BDNF and 5-HT reuptake inhibitors/enhancersx We subsequently test whether strategies that abolish BDNF expression/binding (such as blocking BDNF with antibodies and scavenging peptides or partially eliminating BDNF with transgenic approaches) inhibit the effects of antidepressants on 5-HT fiber sprouting. Finally, we assess the significance of trophic mechanisms for the mediation of antidepressant effects using BDNF +/- mice in laboratory models of depression. In concert, our proposal pursues a novel hypothesis for the effects of antidepressant drugs which has implications for an understanding of mood disorders as disorders of blunted structural plasticity of central monoamine systems.